A turning insert of the type generally mentioned above is previously known by U.S. Pat. No. 4,411,565.
Turning of workpieces of metal or composites is a machining method that is used to manufacture mass-produced, inexpensive products as well as more unique and expensive products. Among other things, thin-walled rings included in jet engines for, for instance, airplanes belong to the latter category. Such rings are assembled with more rings to form a combustion chamber in the engine, and may have a diameter of 2000 mm or more, a width of approx. 100 mm, as well as a thickness of 4-5 mm. The material of the ring has to be heat-resistant and hard, and therefore the same is in practice difficult to machine. A requirement difficult to master is that the dimensional accuracy of the ring has to be meticulous, at the same time as the turned surfaces must not include any defects at all that risk forming points of crack initiation, which during operation could cause disastrous engine breakdowns. In order to avoid all possible points of crack initiation, it is, among other things, necessary that the individual turning operation, e.g., the machining of the inside of the ring (or the outside thereof), has to be performable in a single pass without any interruptions. Namely, if the turning tool in question, and above all the replaceable turning inserts thereof, would fail during the operation in progress before this has been concluded, there are no practical possibilities to avoid surface defects (so-called marks), which may cause crack formations in the ring. Another requirement is that the completed, generated surface of the ring should be dimensionally accurate and as smooth as possible. Therefore, it is important that the concluding pass (if several passes are required) is carried out in a gentle and predictable way. In this connection, it should be pointed out that the requirements of dimensional accuracy in the turning of rings of the kind in question usually are in the order of 0.01 mm.
From what has been said above, it is seen that double-sided turning inserts, which should be able to fulfill the task in question, have to have a service life that is sufficient in order to be able to machine the individual surface of the ring in a single pass without interruptions. A careful machining requires moreover that the turning insert provides good chip control, above all so far that the removed chips must not contact the generated surface. In other words, the chip has to be directed away from the generated surface and most preferably be broken into smaller fragments.
In order to facilitate the understanding of the nature of the invention, reference is initially made to the accompanying FIGS. 1-3, which schematically illustrate certain phenomena having a particular interest in connection with turning. In FIGS. 1-3, CE designates a cutting edge that has a positive cutting geometry and is delimited between a chip surface CS and a clearance surface CLS. The surfaces CS and CLS meet each other at an acute angle, and therefore the rake angle RA of the cutting edge becomes smaller than 90°. In the example, RA amounts to approx. 15°. The bearing surface SS transforms, via a boundary line BL, into a flank surface FS that slopes toward a bottom B, which forms a transition to the chip surface CS. The distance between the boundary line BL and the cutting edge line of the cutting edge C is designated L. A chip removed by the cutting edge CE is illustrated in a simplified way in the form of an arc line CH.
In all forms of chip removing machining in metal, including turning, the rule applies that the chip “is born curved”, i.e., immediately after the moment of removal, the chip obtains an inherent aim at being curved. The shape of the chip, among others its radius of curvature, is determined by several factors the most important ones of which in connection with turning are the feed of the tool, the rake angle of the cutting edge and the cutting depth in question. After the removal, the chip will move perpendicularly to each infinitesimal part of the cutting edge. If the cutting edge is straight, the chip therefore becomes cross-sectionally flat or rectangular, but if the same is entirely or partly arched, also the chip becomes cross-sectionally entirely or partly arched.
In FIG. 1, it is shown how the chip CH is formed without impinging on the flank surface FS. This means that the chip is developed in an uncontrolled way without being directed. Such a chip most often curls into a long, telephone cord-like screw formation, which among other things may impinge on the generated surface of the workpiece or get entangled in the tool and/or the machine. In the example according to FIG. 1, the level difference H1 between the bearing surface SS and the cutting edge CE—or the height of the flank surface FS above the cutting edge CE—in relation to the distance L is too small for the chip to contact the flank surface FS.
In FIG. 2, a turning insert is shown, in which the level difference H2 between the bearing surface SS and the cutting edge CE (=the height of the flank surface) is considerably greater than in the preceding example, the flank surface FS sloping fairly steeply down toward the transition B to the chip surface CS. This means that the chip CH will dive into the flank surface FS with a great force, more precisely in a lower area of the same. The result of this will be that great amounts of heat are developed in the contact area, at the same time as the turning insert becomes blunt-cutting. In addition, the material of the chip may easily adhere to the flank surface FS, even all the way up to the bearing surface SS. After a certain time of use, also wear damage in the flank surface arises. Therefore, neither the embodiment according to FIG. 2 provides good chip control.
In FIG. 3, an embodiment is shown in which the conditions for good chip control are considerably improved. In this case, the height of the flank surface, i.e., the level difference H3 between the bearing surface SS and the cutting edge CE, is selected in such a way that the chip CH will carefully impinge on the flank surface FS in an upper area closest to the bearing surface SS. In such a way, the generation of heat and the tendencies to adhesion are reduced, whereby the easy-cutting properties of the turning insert are maintained. Not only the fact that the chip impinges on the flank surface FS by a moderate force, but also the fact that the distance between the cutting edge and the point of impact of the chip against the flank surface is greater than in FIG. 2 contributes to the moderate heat generation, whereby the temperature in the hot chip has time to be further reduced. When the level difference between the bearing surface and the cutting edge is selected in an optimal way, as is shown in FIG. 3, a good chip control is accordingly created, as will be described in more detail below.
A great difference between a cutting edge having a positive cutting geometry according to the above and a cutting edge having a negative cutting geometry is that the first-mentioned one lifts out the chip by being wedged in between the same and the generated surface, while the last-mentioned one pushes the chip in front of itself while shearing off the same. Generally, positive cutting edges will therefore be more easy-cutting than negative ones and produce chips having greater radii of curvature than the last-mentioned ones.
The turning insert known by U.S. Pat. No. 4,411,565 may be said to be a universal insert for many varying types of turning operations, and should per se have certain merits in connection with turning in general terms. Thus, in the patent document, it is asserted that the turning insert should be able to work within a wide area of cutting depths and feed rates, respectively, while minimizing heat wear of the turning insert and maximizing its service life. However, in practice, the known turning insert is not suitable for such delicate fine turning operations that allow successful machining of, for instance, jet engine rings of the above-mentioned type, among others for reasons mentioned below.
The continuous cutting edge of the turning insert formed by the nose edge and the two converging main edges according to U.S. Pat. No. 4,411,565 is formed with a negative cutting geometry in so far that the chip surface and the clearance surface, which together delimit the cutting edge, form an angle of 90° with each other, i.e., the rake angle=0°. This means that the cutting edge, on one hand, becomes strong but, on the other hand, considerably more blunt-cutting than a cutting edge having a positive cutting geometry. In addition, the same generates significant amounts of heat. A risk with negative, blunt-cutting cutting edges is furthermore that the same may dig into the machined material and interrupt an initiated pass. Another disadvantage is that the level difference between the reference plane in which the nose edges are situated and the lowest situated points of the main edges is many times greater than the level difference between the reference plane and the support surface of the turning insert. This means that the chip, in particular the part of the same that is removed along the fairly long and deeply situated section of the main edge, will impinge on the flank surface positioned inside the chip surface under appreciable heat generation (cf. FIG. 2 above), which may cause adhesion on the flank surface. Furthermore, the declining section of the main edge transforms into a lowest section via an angled, relatively abrupt bob, which may give rise to abrasive jet wear damage in the main edge.
By U.S. Pat. Nos. 5,000,626; 5,082,401; and 5,249,894 and EP 0730925, double-sided, indexable turning inserts having a plurality of cutting edges are previously known, which individually include a nose edge and two main edges. However, in this case, the main edges are straight and situated in the same plane as the nose edges, and therefore good chip control is not obtained.
The present invention aims at obviating the shortcomings of the turning insert known by U.S. Pat. No. 4,411,565 and others and at providing an improved, double-sided turning insert.
An object of the invention to provide an easy-cutting, double-sided turning insert having a good chip control and a reliable function during a service life, which is sufficient to carry out a pass without interruptions.
Another object of the invention is to provide a turning insert having many usable cutting edges that can be mounted in a stable way in the appurtenant tool irrespective of which side of the turning insert that is active and facing upward. Among other things, the turning insert should minimize the risk of metal particles compromising the stability by entering into the interface between the support surface and the corresponding bearing surface of the tool.